The present invention relates to a continuous extrusion and curing line for elastomeric materials such as dense cellular or sponge rubber and for thermosetting materials generally. More particularly the present invention relates to continuous extrusion and hot air curing lines equipped with at least one selectively movable UHF microwave module.
Synthetic rubber products such as extruded sealing strips, gaskets, glass run channel and the like are conventionally made in continuous production lines by extruding an uncured or green rubber extrudate into a desired cross-sectional shape and then curing the extrudate by heating the extrudate and maintaining it at an elevated temperature until it is cured. Elevated temperatures are maintained by use of in-line hot air ovens. In some production lines UHF microwave pre-heaters are used to supply the initial heat for cross-linking, i.e., for curing of the polymeric extrudate. UHF pre-heaters are also known to be combined with other types of curing medium such as ballotinies. In this case UHF can be located before or in a predetermined cut section of the production line.
Where the desired products are dense rubber or cellular rubber products, the conventional use of UHF pre-heaters in combination with hot air is poorly adapted to the successful manufacture of such products. The poor efficiency of UHF preheaters at low temperatures requires the use of relatively high temperatures in the hot air tunnel to cure the product. The use of relatively high temperatures drives lubricant from the products and utilizes the hot air oven in an inefficient manner because only a small portion of hot air volume can be recycled. UHF energy effectively applies heat to the interior of the extrudate rather than the exterior, and if such heat is applied too early in the process, then undesirable results can occur. For example, where the product is cellular rubber, use of UHF too early in the process line leads to excessive and early blowing with associated problems of extrusion distortion, collapsing of foam cells or excessive open cells. Use of UHF too late in the process leads to lower efficiency of the UHF process. Also, optimal power of UHF cannot be used with satisfactory results when it is applied to an extruded product which is not crosslinked at the start of the procedure and, hence, is highly ductile.
Thus, while use of UHF to induce blowing and/or to accelerate crosslinking may be useful, one of the difficulties in practice is to balance the process with aggravated conditions when using the cure line to vulcanize different formulations of products. Complex factors must be taken into consideration such as thermodynamic action of the UHF, the phenomenon of ionizing of the carbon atoms forming the plasma polarized by the electric field, and the action of the cross-linking phenomenon continuing until the extrusion is cured. The present invention allows one to take these factors into consideration and provide a technique for balancing factors to obtain a desired result.
The advantages of the present invention will become further apparent from the following disclosure taken in conjunction with the accompanying drawings and the appended claims.